Radar level gauge (RLG) systems are in wide use for determining the filling level of a product contained in a tank. Radar level gauging is generally performed either by means of non-contact measurement, whereby electromagnetic signals are radiated towards the product contained in the tank, or by means of contact measurement, often referred to as guided wave radar (GWR), whereby electromagnetic signals are guided towards and into the product by a probe acting as a waveguide. The probe is generally arranged to extend vertically from the top towards the bottom of the tank. The probe may also be arranged in a measurement tube, a so-called chamber, that is connected to the outer wall of the tank and is in fluid connection with the inside of the tank.
The transmitted electromagnetic signals are reflected at the surface of the product, and the reflected signals are received by a receiver or transceiver comprised in the radar level gauge system. Based on the transmitted and reflected signals, the distance to the surface of the product can be determined.
More particularly, the distance to the surface of the product is generally determined based on the time between transmission of an electromagnetic signal and reception of the reflection thereof in the interface between the atmosphere in the tank and the product contained therein. In order to determine the actual filling level of the product, the distance from a reference position to the surface is determined based on the above-mentioned time (the so-called time-of-flight) and the propagation velocity of the electromagnetic signals.
Most radar level gauge systems on the market today are either so-called pulsed radar level gauge systems that determine the distance to the surface of the product contained in the tank based on the difference in time between transmission of a pulse and reception of its reflection at the surface of the product, or systems that determine the distance to the surface based on the frequency difference between a transmitted frequency-modulated signal and its reflection at the surface. The latter type of systems are generally referred to as being of the FMCW (Frequency Modulated Continuous Wave) type.
It may be beneficial and, in some cases, even required by law to regularly test the radar level gauge system to verify the measurement accuracy of the radar level gauge system or to calibrate the radar level gauge system.
Testing could be performed by temporarily removing the radar level gauge system, manually measuring the distance to the surface of the product in the tank, and then comparing the thus obtained distance with that provided by the radar level gauge system.
This is, however, often not desirable for various reasons. For example, the tank may be pressurized and/or the product may be dangerous or sensitive. Furthermore, such testing may be time-consuming, cumbersome and not sufficiently accurate.
When the measurement unit of the radar level gauge system can be detached from the signal propagating device (arranged inside the tank) without breaking the process connection, a waveguide of a known length can be used to test the radar level gauge system for the particular distance corresponding to the length of the waveguide. For some applications it would, however, be desirable to test the performance of the waveguide for the current measurement distance in that particular tank, or across the entire measurement range of the radar level gauge system.
One solution aimed at achieving this is the Levelflex M FMP43 Calibration kit sold by Endress+Hauser (http://www.endress.com). This calibration kit contains a number of coaxial cables of different lengths that can be connected to provide several reference distances up to a maximum distance of 1860 mm.
It would, however, be desirable to be able to test the performance of a radar level gauge system for a larger number of measurement distances as well as for a significantly larger maximum distance.